Step-parenting question.... What would you do?

Absolutely not, no say in my opinion. If the girls mom AND dad both are fine with it then she is out numbered anyway. If the mom was allowing it and both the dad and step mom genuinely didn't agree then I would say there needs to be a discussion about it but the discussion should be between the mom and dad only. Even if mom and step mom were both okay with it and dad was not I would still say discussion between mom and dad. I don't think step mom's opinion makes any difference whether she was for or against it.